Who is Yuriy Nosenko from Case Closed: Lee Harvey Oswald and the Assassination of JFK and what is their importance?

Yuriy Nosenko was a thirty-five year old KGB agent who approached the CIA about defecting to the United State two months after the Kennedy assassination. There were doubts about his authenticity and the CIA held him a prisoner for five years before giving him a job and security clearance. He provided information about Oswald's defection to R